In this communication we discuss compensation processes in magnesium doped GaN epilayers and bulk samples. We show that doping with magnesium very efficiently compensates shallow donor centers present in GaN. The observed effect is by far more efficient than expected from the comparison of concentrations of donor and acceptor centers introduced to a sample. Present study demonstrates a significant enhancement of potential fluctuations in doped samples. We evaluate the influence of p-type doping on these potential fluctuations from Kelwin probe measurements. Large- and small-scale light emission fluctuations are demonstrated. Micro-photoluminescence (PL) study indicates an unusual anti-correlation between the intensities of excitonic and defect-related emission processes in p-type doped structures and also the presence of so-called hot PL. Hot PL was observed only in the case of strongly compensated p-type samples, which we relate to strong potential fluctuations in these samples. We discuss possible mechanisms of compensation processes based on the results of our recent optically detected magnetic resonance investigations.

This work was partly supported by the grant numbers 5 P03B 007 20, 5 P03B 123 21 of KBN and by the DENIS program of European Union (G5RD-CT-2001-00566).